The invention relates to a digital transmission system comprising a transmitter coupled to a receiver through a channel, the receiver comprising an equalizer for deriving a detection signal from an input signal of the receiver, a detector for deriving the detected symbols from the detection signal, and the equalizer comprising an equalization filter for deriving at least two equalization signals, combining means for determining the detection signal which is a combination of equalization signals weighted with weight factors, the equalizer also including adapting means for adapting the weight factors in response to a correction signal belonging to a specific weight factor which correction signal is derived from a first auxiliary signal associated to the detection signal and from a second auxiliary signal associated to a detected symbol.
The invention likewise relates to a receiver for such a system.
A system as defined in the opening paragraph is known from the journal article "Techniques for Adaptive Equalization of Digital Communication Systems" by R. W. Lucky in The Bell System Technical Journal, February 1966.
Transmission systems of this type may be used, for example, for transferring digital symbols through the public telephone network or for reconstructing digital symbols coming from a magnetic tape or disc.
When digital symbols are transmitted via a transmission medium or stored on a recording medium, the symbols to be transmitted or recorded respectively, are converted into analog pulses which are fed to the transmission medium or recording medium respectively, further to be referenced by the term of channel.
Generally, the analog pulses are provided not to overlap in time. If the channel has a limited bandwidth, the pulses will start to overlap which will often lead to the fact that a signal received at a specific instant does not only depend on a single data symbol, but also on symbols adjacent in time. This effect is called intersymbol interference.
In addition to being caused by a limited bandwidth, intersymbol interference may also be caused by the use of a band limiting filter at the transmitter end, which is used to give a desired shape to the frequency spectrum of the transmitted or recorded analog pulses. In many cases the presence of intersymbol interference will lead to an increased symbol error rate.
A possibility of restricting the increase of the symbol error rate caused by intersymbol interference is the use of an equalizer either adaptive or not. An adaptive equalizer may comprise, for example, a filter having an adjustable transfer, which is inserted between the input of the receiver and the detector. The transfer function for such an adaptive equalizer is adjusted so that the error criterion is minimized. In practice a variety of error criteria is used.
A first error criterion is the minimized mean square error. For this criterion the mean square value of the difference between the detection signal and the detected symbol value is minimized. This is equivalent to minimizing the sum of the intersymbol interference power and the noise power at the input of the detector. A second error criterion is the minimized mean square distortion which is equivalent to a minimization of the intersymbol interference power. Both criteria are equivalent if the noise power is negligibly small.
An error criterion known from said journal article is the so-termed zero forcing criterion, for which a number of values of the impulse response of the combination of channel and equalizer is adjusted to a predetermined desired value. As a result, this impulse response becomes equal to a desired impulse response g(t) within a certain period of time.
In the transmission system known from said journal article the equalizer comprises an adaptive transversal filter whose coefficients are adapted in response to the correction signal, the correction signal being derived from the first and second auxiliary signals. The equalization filter then comprises a plurality of delay elements and the detection signal is obtained by calculating the sum of the equalization filter output signals weighted with a weight factor. For all the equalizer coefficients the first auxiliary signal associated to the detection signal is the difference between the detection signal and the most recently detected symbol value which difference is delayed by one symbol interval, whereas for the different coefficients the second auxiliary signal is formed by detected symbol values delayed over zero, one and two symbol intervals. The zero forcing criterion is used in the equalizer known from said journal article.
The MMSE criterion compared with the zero forcing criterion is advantageous in that convergence to a correct equalizer setting will nearly always occur, whereas, the convergence of the equalizer, if the zero forcing criterion is used, is only guaranteed if the eye pattern for the equalizer is open. In addition, if the MMSE criterion is used, no excessive noise enhancement occurs in channels with spectral zero points, whereas this may occur indeed with the zero forcing criterion. In the commonly used LMS algorithm the MMSE criterion is used.
The zero forcing criterion relative to the MMSE criterion is advantageous in that it is simpler to implement and in that it is less sensitive to changes of the channel gain factor. In addition, with the zero forcing criterion it does not happen that coefficients of the equalizer become very large (coefficient blow up) which may happen in some cases if the MMSE criterion is used.